1. Field of the Invention
The present invention relates generally to a handoff in a cellular network, and more particularly to eliminating an audio gap during a hand-off.
2. Description of the Background Art
Cellular phones and cellular phone networks have fairly recently become a widespread phenomenon. To many people, cell phones have become an essential communications tool to be used for business, family, travel, security, and entertainment communications purposes. Cell phones have become so commonplace that they are almost taken for granted.
The driving reason behind the popularity of cell phones is their mobility. People can use them almost anywhere, in a car, taxi, bus, or train, at home, at work, in restaurants or motels, while taking a walk, etc. This mobility is achieved through use of multiple base stations which communicate with cell phones using a radio frequency (RF) transmission, and which also communicate with the public switched telephone network (PSTN). Each base station has an assigned region of coverage known as a cell. When a mobile phone user moves from one cell to another, the base station must xe2x80x9chand-offxe2x80x9d or transfer the call to the base station assigned to the other cell.
FIG. 1 shows a representative cell pattern, illustrating how a cell phone network achieves approximately continuous coverage. Each cell A, B, C, etc., represents a substantially circular coverage pattern generated by a cellular base station situated at the center of each circle. Due to overlap, a mobile station moving through the region is generally able to maintain contact with the cellular network to conduct a call.
A moving mobile station, for example moving from cell C to cell H, must be handed-off from base station C to base station H in order to maintain the call. This needs to be done without any interruption of service to the user. Therefore, the cellular network first must detect a need for a hand-off, then find a suitable target base station (determined by the direction of the motion of the mobile station), drop the current base station, and switch the call to the target base station. All of this is desirably done without audio gaps in the call.
FIG. 2 shows a typical CDMA (Code Division Multiple Access) cellular network system having a mobile switching center (MSC), one or more cellular base site controllers (CBSCs), and a plurality of base station transceivers (BTSs).
The MSC functions to route calls among the BTSs A-D, and also functions to interconnect the cellular network to a local public switched telephone network (PSTN). Through the PSTN, the MSC may be connected to geographically remote MSCs as well as to PSTN central switching offices.
A cellular network is typically constructed having a plurality of BTSs. These BTSs are under control of a CBSC. The CBSC interacts with a mobile station (i.e., a cellular phone) via a BTS to determine a suitable BTS for a call, and establishes a connection to the mobile station. A CBSC also provides message transfer and call switching as directed by the MSC. Control information received from an mobile station (through a BTS and a CBSC) is routed to the MSC. Calls received by a CBSC from the MSC, on a channel of a communication link between the MSC and CBSC, are switched under control of the CBSC to the appropriate BTS in communication with the mobile station. Routing of all calls (including those between mobile stations under the same CBSC) are routed through the MSC.
In order to properly conduct a hand-off, the MSC, the old CBSC and the old BTS, the target CBSC and the target BTS, and the mobile station must interchange data so that a call is routed through a new call path and also so that the old call path is terminated at a correct time. As part of the hand-off, and as part of normal operation, a mobile station stores an active BTS set and a target BTS set. The active BTS set is a set of all BTSs currently communicating with the mobile station (more than one BTS may be communicating, as in BTS A and BTS B, within a cell and common to a CBSC). The active BTS set therefore dictates what BTSs the mobile station transmits to and receives from. The target BTS set stores BTSs that, due to signal strength measurements, have been determined by the old CBSC to be handoff candidates.
Prior to a hand-off, the mobile station may transmit the contents of the target BTS set to the controlling CBSC (the old CBSC) so that the old CBSC may determine if a handoff should occur. In addition, the old CBSC may use the target BTS set to determine a target BTS. During the hand-off, the target BTS set may be inserted into the active BTS set by the old CBSC as part of creating the new call path.
It should be noted that switching between BTSs common to a CBSC is generally not a problem, as BTSs under a common CBSC are capable of simultaneously transmitting a call. Therefore, a mobile station traveling between BTS B and BTS A, for example, may be switched by the (common) CBSC, without the need for a handoff between CBSCs.
In the prior art, a handoff has typically been done in a straightforward and simple manner. FIGS. 2-6 are schematic diagrams showing a typical prior art handoff sequence. In FIG. 2, the MSC is conducting the call through a CBSC A and BTS B (the old BTS). In this example, the mobile station is traveling from the cell site of BTS B into the cell site of BTS C. The mobile station, by measuring signal strengths of signals received from various local BTSs, including BTS B and BTS C, is able to determine (or suggest) that a handoff should occur. Likewise, by using measured signal strengths, the mobile station is able to determine (or suggest) that the handoff should be from BTS B to BTS C.
In FIG. 3, as a second step in a prior art hand-off, a new call path is created. The new call path extends through the target CBSC B and the target BTS C to the mobile station. It should be noted that at this point in time the communication between the mobile station and the target BTS C is limited to a one-way communication, traveling only from the mobile station to the target BTS C.
In FIG. 4, as a third step, a Handoff Direction Message has just been sent from the old CBSC A to the mobile station. As a result, both the old CBSC A and the target CBSC B have been removed from the active BTS set of the mobile station until the mobile station reacquires the BTSs. The effect is that both the old call path and the new call path are temporarily suspended.
In FIG. 5, as a fourth step, the mobile station reacquires the connection to the CBSC B and to the BTS C. At this time, two way communication is initiated between the mobile device and the target BTS C, and only one way communication exists from the mobile device to the old BTS B.
In FIG. 6, as a fifth step, the handoff is completed and the old call path is terminated.
It should be noted that each step above may require a time period to accomplish. As can be seen from the sequence of FIGS. 2-5, a new call path is created and then as part of the switching both the old and new call paths to/from the mobile station are temporarily terminated (the call paths are re-established when the mobile station reacquires both BTSs). The result is an audio gap, wherein a portion of the transmitted signal, such as speech, for example, is simply lost. A typical resulting audio gap is on the order of about 250 milliseconds. This is very undesirable.
While this audio gap is troublesome and annoying in voice communications, the audio gap may have even more troublesome effects in data communications. Wireless modems have become increasingly common as people use devices such as laptop computers, personal digital assistants (PDAs), text-capable pagers, etc., to send and receive digital data communications in a wide variety of settings. For example, it is increasingly common for commuters to use wireless devices to access e-mail and perform stock trades, for example.
While wireless modems are a great convenience in many ways, they do not handle audio gaps well. A 250 millisecond audio gap may cause a wireless modem to lose data, receive corrupted data, or lose the connection entirely. This weakness is present because if a packet data frame is missing or truncated, reception of other data packets may be affected.
An additional drawback may result when hand-offs repeatedly occur. For example, if a user is moving so that he or she is traveling in an overlap region between adjacent cell sites, the mobile station may be repeatedly switched between the two cell sites. Therefore, the user may be subjected to multiple and ongoing audio gaps.
Prior art efforts to address this problem have included the addition of linkages between CBSCs, such as a land line. However, this is an expensive solution that may require additional land lines and modifications to existing hardware.
What is needed, therefore, are improvements in hand-offs in a cellular network.